Transcript Concept 7.1: Cellular membranes are fluid mosaics of

Concept 7.1: Cellular membranes are
fluid mosaics of lipids and proteins
• Phospholipids are the most abundant lipid in
the plasma membrane
• Phospholipids are amphipathic molecules,
containing hydrophobic and hydrophilic regions
• The fluid mosaic model states that a membrane
is a fluid structure with a “mosaic” of various
proteins embedded in it
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Membrane Models: Scientific Inquiry
• Membranes have been chemically analyzed and
found to be made of proteins and lipids
• Scientists studying the plasma membrane
reasoned that it must be a phospholipid bilayer
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Fig. 7-2
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Fig. 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
• Peripheral proteins are bound to the surface of
the membrane
• Integral proteins penetrate the hydrophobic
core
• Integral proteins that span the membrane are
called transmembrane proteins
• The hydrophobic regions of an integral protein
consist of one or more stretches of nonpolar
amino acids, often coiled into alpha helices
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Fig. 7-9
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Glycoprotein
(d) Cell-cell recognition
The Permeability of the Lipid Bilayer
• Hydrophobic (nonpolar) molecules, such as
hydrocarbons, can dissolve in the lipid bilayer
and pass through the membrane rapidly
• Polar molecules, such as sugars, do not cross the
membrane easily
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Concept 7.3: Passive transport is diffusion of
a substance across a membrane with no
energy investment
• Diffusion is the tendency for molecules to
spread out evenly into the available space
• Although each molecule moves randomly,
diffusion of a population of molecules may
exhibit a net movement in one direction
• At dynamic equilibrium, as many molecules
cross one way as cross in the other direction
Animation: Membrane Selectivity
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Animation: Diffusion
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium
• Substances diffuse down their concentration
gradient, the difference in concentration of a
substance from one area to another
• No work must be done to move substances
down the concentration gradient
• The diffusion of a substance across a biological
membrane is passive transport because it
requires no energy from the cell to make it
happen
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Effects of Osmosis on Water Balance
• Osmosis is the diffusion of water across a
selectively permeable membrane
• Water diffuses across a membrane from the
region of lower solute concentration to the
region of higher solute concentration
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Fig. 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis
Same concentration
of sugar
Water Balance of Cells Without Walls
• Tonicity is the ability of a solution to cause a cell
to gain or lose water
• Isotonic solution: Solute concentration is the
same as that inside the cell; no net water
movement across the plasma membrane
• Hypertonic solution: Solute concentration is
greater than that inside the cell; cell loses water
• Hypotonic solution: Solute concentration is less
than that inside the cell; cell gains water
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Fig. 7-13
Hypotonic solution
H2O
Isotonic solution
Hypertonic solution
H2O
H2O
H2O
(a) Animal
cell
Lysed
H2O
Normal
Shriveled
H2O
H2O
H2O
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed
• Hypertonic or hypotonic environments create
osmotic problems for organisms
• Osmoregulation, the control of water balance,
is a necessary adaptation for life in such
environments
• The protist Paramecium, which is hypertonic to
its pond water environment, has a contractile
vacuole that acts as a pump
Video: Chlamydomonas
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Video: Paramecium Vacuole
Fig. 7-14
Filling vacuole
50 µm
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.
Water Balance of Cells with Walls
• Cell walls help maintain water balance
• A plant cell in a hypotonic solution swells until
the wall opposes uptake; the cell is now turgid
(firm)
• If a plant cell and its surroundings are isotonic,
there is no net movement of water into the cell;
the cell becomes flaccid (limp), and the plant
may wilt
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• In a hypertonic environment, plant cells lose
water; eventually, the membrane pulls away
from the wall, a usually lethal effect called
plasmolysis
Video: Plasmolysis
Video: Turgid Elodea
Animation: Osmosis
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Facilitated Diffusion: Passive Transport
Aided by Proteins
• In facilitated diffusion, transport proteins speed
the passive movement of molecules across the
plasma membrane
• Channel proteins provide corridors that allow a
specific molecule or ion to cross the membrane
• Channel proteins include
– Aquaporins, for facilitated diffusion of water
– Ion channels that open or close in response to a
stimulus (gated channels)
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Fig. 7-15
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Carrier protein
(b) A carrier protein
Solute
Concept 7.4: Active transport uses energy to
move solutes against their gradients
• Facilitated diffusion is still passive because the
solute moves down its concentration gradient
• Some transport proteins, however, can move
solutes against their concentration gradients
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The Need for Energy in Active Transport
• Active transport moves substances against their
concentration gradient
• Active transport requires energy, usually in the
form of ATP
• Active transport is performed by specific
proteins embedded in the membranes
Animation: Active Transport
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• Active transport allows cells to maintain
concentration gradients that differ from their
surroundings
• The sodium-potassium pump is one type of
active transport system
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Fig. 7-16-7
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
1
Na+
[Na+] low
[K+] high
P
ADP
2
ATP
P
3
P
P
6
5
4
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Concept 7.5: Bulk transport across the
plasma membrane occurs by exocytosis
and endocytosis
• Small molecules and water enter or leave the
cell through the lipid bilayer or by transport
proteins
• Large molecules, such as polysaccharides and
proteins, cross the membrane in bulk via
vesicles
• Bulk transport requires energy
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Exocytosis
• In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their
contents
• Many secretory cells use exocytosis to export
their products
Animation: Exocytosis
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Endocytosis
• In endocytosis, the cell takes in macromolecules by
forming vesicles from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving
different proteins
• There are three types of endocytosis:
– Phagocytosis (“cellular eating”)
– Pinocytosis (“cellular drinking”)
– Receptor-mediated endocytosis
Animation: Exocytosis and Endocytosis Introduction
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• In phagocytosis a cell engulfs a particle in a
vacuole
• The vacuole fuses with a lysosome to digest the
particle
Animation: Phagocytosis
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Fig. 7-20
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
Pseudopodium
of amoeba
“Food”or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs)
Coat
protein
Plasma
membrane
0.25 µm